1. Field of Invention
This invention pertains to an apparatus and method for in situ detection of radiation. More particularly, the invention pertains to in situ detection of radiation in limited access areas such as well-logging bore holes.
2. Description of Prior Art
The importance of rapid, precise, real-time measurement and surveillance of radiation-emitting substances such as radionuclides that have the potential for severe adverse effects on public health cannot be overemphasized. Where it is known that such substances exist in waste disposal sites or where it is suspected that the substances may have intruded into underground aquifers or public water supplies for example, it is imperative that continuous, real-time, precise tracking of the substances be obtained. Such surveillance and sampling should also permit remote and automated operation and should not further complicate the problem by the accumulation of large numbers of contaminated samples requiring proper storage and disposal and always at risk of potential mislabeling by field personnel. Additionally, the sample apparatus and procedure should not affect the environment being measured such that the reliability of the sample is assured.
Four principle types of electromagnetic radiation are generated by the radiation-emitting substances for which surveillance is desired. A first type is bremsstrahlung radiation which is produced by the deceleration or acceleration of charged particles such as when an electron is near the field of an atomic nucleus. Another type is fluorescence which occurs when a substance emits radiation of one frequency when exposed to radiation of another frequency. A third type of radiation occurs from the phenomena known as the Compton effect in which the wavelength of X-ray or gamma-ray photons is increased as a result of these photons striking an electron. Cerenkov radiation, the fourth type, is produced when charged particles pass through a transparent solid or liquid medium faster than the speed of light in the same medium. Of the four types, Compton effect radiation and fluorescence require the presence of external excitation radiation whereas bremsstrahlung and Cerenkov radiation result from the presence of charged particles, for example beta particles, inherent in the sample itself and have frequencies specific to the substance emitting the charged particle. Therefore the latter two types of radiation are much more easily adapted to in situ measurement of radiation in locations where access is restricted or difficult.
Two principle approaches have been followed in the monitoring and measurement of beta radiation in aqueous samples. Evaporation is the usual method for preparing aqueous samples for measurement of beta radiation. Major disadvantages exist, however, in that the samples must be taken to a laboratory for preparation and radiation losses occur from the adsorption on evaporation vessels and from the volatility of some compounds. Additionally, the samples may take weeks or months to process. Using bremsstrahlung radiation, the second approach, requires compilation of libraries of reference spectra of all background contributions from other radiation sources in order to detect the radionuclide emitting the beta particles. Such a library is specific to the many parameters of the location in which sampling is conducted. The present invention, on the other hand, uses the principle of Cerenkov radiation in a sampling device which avoids all the limitations of the abovementioned conventional approaches in that (1) there is no necessary sample preparation or transport; (2) no sample losses occur from evaporation or adsorption; (3) no reference library of spectra need be developed specific to the test site in order to ascertain the presence of a beta-emitting radionuclides; (4) the test site is not disturbed by the sample procedure; and (5 ) no provision must be made for the collection, storage and disposal of contaminated samples.
The in situ detector of the present invention permits in-place screening of fluid media for radiation emitting substances, especially radioactive contaminants, and sample analysis in real time. By way of example and not limitation, screening of radioactive energetic beta/gamma ray contamination can be accomplished within approximately one-half hour of monitoring which is orders of magnitude faster than any presently available technology. The detector may also be permanently emplaced with remote-sensing capability and computer-controlled processing, which allows for rapid assessment of contaminant concentration and migration, data storage and retrieval and no necessity for exposure of person(s) to risks inherent at the sample site.